Stents are well known medical devices that are used for maintaining the patency of a large variety of vessels of the human body. A more frequent use is for implantation into the coronary vasculature. Although stents have been used for this purpose for more than ten years, and some current stent designs such as the CORDIS BX Velocity(copyright) stent, Cordis Corporation, Miami Lakes, Fla., have the required flexibility and radial rigidity to provide an excellent clinical result, they are not always clearly seen under standard fluoroscopy.
Many current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting links or undulating longitudinal connecting links. The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved sections forming a closed-ring, zig-zag structure. This structure opens up as the stent expands to form the element in the stent that provides structural support for the arterial wall. A single strut member can be thought of as a diagonal section connected to a curved section within one of the circumferential sets of strut members. In current stent designs such as the BX Velocity(copyright) stent, these sets of strut members are formed from a single piece of metal having a uniform wall thickness and generally uniform strut width. Although a stent with uniform width of the strut members will function, if the width is increased to add strength or radiopacity, the sets of strut members will experience increased strain upon expansion. High strain can cause cracking of the metal and potential fatigue failure of the stent under the cyclic stress of a beating heart.
Existing highly radiopaque stents, such as the gold plated NIROYAL stent sold by Boston Scientific, Inc., Natick Mass., can obscure the inside of the vessel due to the high radiopacity over the entire length of the stent. The BeStent sold by Medtronic, Inc., Minneapolis Minn., has small gold markers at the ends of the stent. Those markers only mark an end point without allowing visualization of the entire end set of strut members.
Fischell et al, in U.S. Pat. No. 6,086,604, discloses a stent with the end sets of strut members being gold plated. Such a stent would have ideal radiopacity but may be subject to the corrosive effects incurred through placement of dissimilar metals in an electrolytic solution such as blood. There has also been significant evidence that gold is a poor surface material for stents because it may increase the risk of subacute thrombosis or restenosis. Further, Fischell et al, in U.S. Pat. No. 5,697,971 discloses in its FIG. 7, a stainless steel stent with increased width diagonal sections in all the circumferential sets of strut members.
An ideally radiopaque stent would have end sets of strut members that are highly radiopaque so that they can be readily seen, even using low power fluoroscopy, and would further contain a central section that is visible but not too bright so as to obscure the lumen when high power cine film angiograms are taken. The stent should also have only one material on its outside surface to avoid potential corrosion; that material should not promote subacute thrombosis or restenosis.
The present invention is a stent that is designed to have optimal strength and radiopacity with good biocompatibility. Unfortunately, the choices of appropriate biocompatible metals available as thin wall tubing for stent construction are somewhat limited. To achieve optimal radiopacity, the stent design of the present invention is adjusted to the specific radiopacity and strength characteristics of the metal from which the stent is fabricated. What is more, coatings such as parylene may be needed to avoid corrosion from stents with less biocompatible materials and/or dissimilar metals on the stent""s outer surface. Of extreme importance to the present invention is the achievement of optimal radiopacity in a stent that ideally is only 0.004 inches wall thickness or less. Such a stent would have a pre-deployment outer diameter (profile) that would be at least 0.003 inches less than currently marketed stents. Ideally, the stent described herein would have a wall thickness between 0.0025 inches and 0.004 inches.
Described herein are the novel design elements for stents formed from the following materials:
1. A highly radiopaque metal such as tantalum;
2. Metals somewhat more radiopaque than stainless steel, such as the cobalt based alloy L605;
3. Stents coated or plated with highly radiopaque materials like gold; and
4. Layered materials such as alternative layers of tantalum and stainless steel.
5. The novel design elements that are described herein include:
1. Tapered Strut Width for Stents Formed from Highly Radiopaque Metals.
Although reducing the width of the longitudinally diagonal section alone will reduce radiopacity without significantly affecting radial strength, by having a taper on the curved sections of the circumferential sets of strut members, a greatly reduced level of strain upon stent expansion can be achieved without sacrificing radial strength. This is extremely important, as it allows a stent to be made much stronger than a stent with uniform width of the strut members while staying within the same strain limit for the material.
Tantalum is a metal that has been used in stents; which metal is highly radiopaque. The optimal radiopacity for a stent design using tantalum could have uniform width for the circumferential sets of strut members and a wall thickness of about 0.0025 inches. To provide more radial strength and to reduce the probability of the stent ends flaring out during deployment, a wall thickness of about 0.003 inches to 0.035 inches would be highly desirable. With uniform width sets of strut members, a 0.035 inches wall thickness tantalum stent would be too bright under cine angiography. To reduce the radiopacity of the design without significantly impacting the radial strength of the deployed stent, the present invention envisions curved sections and diagonal sections, either or both of which could have a variable or tapered width. The curved sections should be tapered (wider at the center compared to the ends) to reduce strain as previously described. The longitudinally diagonal sections can be thinner in the center than at the ends, to reduce radiopacity for the central sets of strut members.
It is envisioned that the novel stent described herein might have wider diagonal sections for the end sets of strut members as compared to the central sets of strut members. This feature would enhance the radiopacity of the end sets of strut members while retaining a moderate level of radiopacity for the central sets of strut members. It is also envisioned to have both reduced width diagonals and/or reduced wall thickness for the central sets of strut members. It should be remembered that it is fluoroscopic visualization of the end sets of strut members that is most important for visualizing stents placed inside a coronary artery.
2. Thicker Diagonal Sections for Metals with Radiopacity Slightly Better than Stainless Steel.
The cobalt/tungsten alloy L605 is a stronger and more radiopaque metal compared to stainless steel. To achieve optimal radiopacity using L605 with uniform width sets of strut members, the wall thickness is optimally equal to or greater than 0.0045 inches. To provide optimal radiopacity with such a metal in stents of wall thickness 0.004 inches or less, the present invention envisions wider diagonal sections in the sets of strut members. Thus, the tapered diagonal sections would be wider than the curved sections. The tapered curved section design for reduced strain may also be highly desirable for stents made from the L605 alloy.
3. End Sets of Strut Members with Thinner Curved Sections.
Stent deliverability into curved coronary arteries is improved when the diagonal sections of the end sets of strut members have a decreased length as compared to the length of the diagonal sections of the central sets of strut members. A shorter length of the diagonal sections will also reduce outward flaring upon expansion of the stent. Decreasing end flaring of the deployed stent is of particular importance for stents having very thin walls.
Previous designs that describe a stent with shorter diagonal sections in the end sets of strut members are limited by the strain limit allowed for the end sets of strut members. As a result, if the end sets of strut members are made as strong as possible while being limited by the maximum allowable strain for that metal, the central sets of strut members will not have optimized radial strength. The present invention envisions optimizing the radial strength for all sets of strut members, i.e., the metal in all sets of strut members just reach the maximum allowable strain at the limiting diameter for the stent""s expansion. To achieve this desired attribute, the stent described herein has the curved sections of the end sets of strut members being less wide than the curved sections of the central sets of strut members.
4. Good Side Branch Arterial Access While Maintaining Small Cell Size.
The stents described herein are typically closed cell stents, having a curved section of a central set of strut members connected to an adjacent set of strut members by a longitudinally extending link. In one embodiment of the present invention, the circumferential sets of strut members are joined by undulating longitudinal connecting links with each link having a multiplicity of curved segments so as to increase the perimeter of the stent""s closed cells. One aspect of the present invention is that the perimeter of each of the stent""s closed cells should be at least 9 mm long. This design parameter allows each cell of the stent to be expanded to a circular diameter of approximately 3 mm (i.e., 9/xcfx80 mmxcx9c3 mm). This feature allows the xe2x80x9cunjailingxe2x80x9d of side branches of the artery into which the stent is placed. The ideal design to be radially strong, prevent plaque prolapse and still allow sidebranch access will have a maximum deployed cell area of less than 0.005 in.2 while having a cell perimeter that is at least 9 mm in length, so as to allow unjailing of side branches. A good cell for side branch access should have a perimeter length between 9 mm and 11 mm. (i.e. an expandable circular diameter between 2.86 mm and 3.5 mm). Cell perimeters between 9.5 and 10 mm are optimal.
5. Flexible Undulating Longitudinal Links with Good Support Between Adjacent Sets of Strut Members.
To provide a strong bridge connection between adjacent circumferential sets of strut members, the flexible undulating longitudinal connecting links should have nearly equal extension in the circumferential direction on each side of a line drawn between the attachment points of the flexible undulating longitudinal connecting link to the curved sections of adjacent sets of strut members. xe2x80x9cNxe2x80x9d and inverted xe2x80x9cNxe2x80x9d shapes for the connecting links inherently have equal circumferential displacement on each side of the line connecting their attachment points. The specially designed xe2x80x9cMxe2x80x9d or xe2x80x9cWxe2x80x9d shapes of the present invention also provide this desirable attribute. Nearly equal circumferential lengths on either side of a line drawn between the attachment points of the flexible undulating longitudinal connecting links help in preventing plaque from pushing the xe2x80x9cMxe2x80x9d or xe2x80x9cWxe2x80x9d shaped link inward into the lumen of the stent when the stent is deployed into an artery.
The xe2x80x9cMxe2x80x9d and xe2x80x9cWxe2x80x9d shapes are of particular advantage in obtaining the desired attribute of small area cells that have good side branch access capability because of an increased perimeter length. It should also be understood that the xe2x80x9cMxe2x80x9d and xe2x80x9cWxe2x80x9d shapes each add an additional half cycle of undulating link length to the cell perimeter as compared to an xe2x80x9cNxe2x80x9d shaped link design, thus improving the stent""s longitudinal flexibility. It should also be noted that a xe2x80x9cWxe2x80x9d link is simply an inverted xe2x80x9cMxe2x80x9d link.
6. Variable Thickness Radiopaque Coatings.
The NIROYAL(trademark) stent has a uniform thickness of gold plating, which makes the center too radiopaque as compared to the radiopacity of the end sets of strut members. Fischell et al., U.S. Pat. No. 6,086,604, teaches stents having gold placed at the end sets of strut members. This creates a potential for corrosion from dissimilar metals, namely, gold and stainless steel. The present invention envisions a gold coating that is sufficiently thick on the end sets of strut members to provide optimal radiopacity with a thin coating of gold on the rest of the stent. This design prevents obscuring of the arterial lumen while providing an exterior surface for the stent that is a single metal, thus avoiding electrolytic corrosion.
7. Polymer Coatings for Stents Coated with Gold or Having Dissimilar Metal Surfaces.
For stents with non-biocompatible or dissimilar metals, the present invention envisions the use of a polymer such as parylene to coat the entire outer surface of the stent. This would improve biocompatibility and also allow attachment of organic compounds such as heparin or phosphorylcholine for reduced thrombogenicity or drugs, such as taxol or rapamycin, for reduced cell proliferation and a decreased rate of restenosis. It is also known that highly radiopaque materials like tungsten can be mixed into polymers. A stent coating including a plastic with mixed in radiopaque metal could be used to enhance both radiopacity and biocompatibility. Such a polymer coating could also be advantageous with a gold coated stent.
8. Providing a Variable Wall Thickness.
The present invention also envisions next generation manufacturing techniques using photo-etching, whereby a stent pattern is etched into a thin-walled metal tube. These techniques already can produce variations in wall thickness as well as strut width for any stent pattern. The present invention envisions use of these techniques to create stents with optimal radiopacity. In particular for a stent formed from a single metal or alloy, thicker metal at each end of the stent could increase radiopacity there as compared to the central section of the stent. Perhaps more important is the use of multi-thickness etching techniques with a two- or three- layered tube where one of the layers is a highly radiopaque material such as tantalum. For example, a two-layer tube having one layer of stainless steel and a second layer of tantalum could be etched to provide the end sets of strut members with 0.001 inches of tantalum and 0.0025 inches of stainless steel while the remainder of the stent would have less than 0.0005 inches of tantalum with a stainless steel layer of 0.003 inches. It is also envisioned that there could be tantalum only on the end sets of strut members. Thus, one could produce a stent with enhanced radiopacity at the ends with the stent having a uniform wall thickness.
One could even have a stent with increased wall thickness of a metal at the central region of the stent but still having a decreased radiopacity at that central region if, for example, the stent had tantalum end struts with stainless steel center struts. Such a stent would be strongest in the center where the thickest plaque must be restrained.
It is also envisioned that any of the above optimal radiopacity stent designs may be used with plastic coatings such as parylene, antithrombogenic coatings such as heparin or phosphorylcholine, or anti-proliferative coatings such as taxol or rapamycin.
Thus it is an object of the present invention to have a stent with tapered curved sections, the center of the curved sections being wider than ends of the curved sections so as to reduce plastic strain as the stent is expanded as compared to a curved section with uniform width.
Another object of the present invention is to have a stent with tapered diagonal sections in the sets of strut members where the center of the diagonal section is narrower than the ends to reduce the radiopacity of central sets of strut members of the stent as compared to a stent with diagonal sections having a uniform width.
Still another object of the invention is to have a stent with decreased wall thickness at the central struts compared to the end struts so as to have a comparatively higher radiopacity for the end sets of strut members.
Still another object of the present invention is to have a stent with tapered diagonal sections for one or more of the sets of strut members where the center of the diagonal section is wider than the ends to increase the radiopacity of the end sets of strut members as compared to a stent with uniform width of the diagonal sections.
Still another object of the present invention is to have end sets of strut members having both shorter diagonal sections and thinner width curved sections as compared to those sections in the central sets of strut members.
Still another object of the present invention is to have a tantalum stent with wall thickness less than 0.035 inches having tapered sets of strut members whereby the diagonal sections are less wide than the width at the center of the curved sections.
Still another object of the present invention is to have a closed cell stent design with maximum post-deployment cell area less than 0.005 square inches and a cell perimeter length that is equal to or greater than 9 mm.
Still another object of the present invention is to have a stent with a radiopaque metal coating where the radiopaque metal coating has greater wall thickness on the end sets of strut members as compared to thickness on the sets of strut members at the center of the stent.
Still another object of the present invention is to have a stent etched from a multi-layer metal tube having one layer significantly more radiopaque than at least one other layer; the etched stent being formed with increased wall thickness of the more radiopaque layer on the end sets of strut members as compared with the sets of strut members at the center of the stent.
Still another object of the present invention is to have a closed cell stent design with xe2x80x9cMxe2x80x9d or xe2x80x9cWxe2x80x9d shaped flexible undulating longitudinal connecting links wherein the circumferential extent of the flexible undulating longitudinal connecting links is approximately equal on each side of a line drawn between the proximal and distal attachment points of the flexible undulating longitudinal connecting link.
Still another object of the present invention is to have the stent with optimized radiopacity formed with an outer surface that is plastic coated to improve biocompatibility.
Still another object of the present invention is to have the stent with optimized radiopacity that is coated with a plastic material and an additional organic compound to prevent thrombus formation and/or restenosis.
Still another object of the present invention is to have a stent coated with a plastic material that includes a radiopaque filler material.
These and other objects and advantages of this invention will become apparent to the person of ordinary skill in this art field upon reading of the detailed description of this invention including the associated drawings.